/* eslint-disable indent */
import { escapeText } from './utilities.js';

/*
 * This file is a modified version of google-diff-match-patch's JavaScript implementation
 * (https://code.google.com/p/google-diff-match-patch/source/browse/trunk/javascript/diff_match_patch_uncompressed.js),
 * modifications are licensed as more fully set forth in LICENSE.txt.
 *
 * The original source of google-diff-match-patch is attributable and licensed as follows:
 *
 * Copyright 2006 Google Inc.
 * https://code.google.com/p/google-diff-match-patch/
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * https://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 * More Info:
 *  https://code.google.com/p/google-diff-match-patch/
 *
 * Usage: QUnit.diff(expected, actual)
 *
 */
  function DiffMatchPatch () {
  }

  //  DIFF FUNCTIONS

  /**
   * The data structure representing a diff is an array of tuples:
   * [[DIFF_DELETE, 'Hello'], [DIFF_INSERT, 'Goodbye'], [DIFF_EQUAL, ' world.']]
   * which means: delete 'Hello', add 'Goodbye' and keep ' world.'
   */
  const DIFF_DELETE = -1;
  const DIFF_INSERT = 1;
  const DIFF_EQUAL = 0;
  const hasOwn = Object.prototype.hasOwnProperty;

  /**
   * Find the differences between two texts.  Simplifies the problem by stripping
   * any common prefix or suffix off the texts before diffing.
   * @param {string} text1 Old string to be diffed.
   * @param {string} text2 New string to be diffed.
   * @param {boolean=} optChecklines Optional speedup flag. If present and false,
   *     then don't run a line-level diff first to identify the changed areas.
   *     Defaults to true, which does a faster, slightly less optimal diff.
   * @return {!Array.<!DiffMatchPatch.Diff>} Array of diff tuples.
   */
  DiffMatchPatch.prototype.DiffMain = function (text1, text2, optChecklines) {
    // The diff must be complete in up to 1 second.
    let deadline = Date.now() + 1000;

    // Check for null inputs.
    if (text1 === null || text2 === null) {
      throw new Error('Cannot diff null input.');
    }

    // Check for equality (speedup).
    if (text1 === text2) {
      if (text1) {
        return [
          [DIFF_EQUAL, text1]
        ];
      }
      return [];
    }

    if (typeof optChecklines === 'undefined') {
      optChecklines = true;
    }

    // Trim off common prefix (speedup).
    let commonlength = this.diffCommonPrefix(text1, text2);
    let commonprefix = text1.substring(0, commonlength);
    text1 = text1.substring(commonlength);
    text2 = text2.substring(commonlength);

    // Trim off common suffix (speedup).
    commonlength = this.diffCommonSuffix(text1, text2);
    let commonsuffix = text1.substring(text1.length - commonlength);
    text1 = text1.substring(0, text1.length - commonlength);
    text2 = text2.substring(0, text2.length - commonlength);

    // Compute the diff on the middle block.
    let diffs = this.diffCompute(text1, text2, optChecklines, deadline);

    // Restore the prefix and suffix.
    if (commonprefix) {
      diffs.unshift([DIFF_EQUAL, commonprefix]);
    }
    if (commonsuffix) {
      diffs.push([DIFF_EQUAL, commonsuffix]);
    }
    this.diffCleanupMerge(diffs);
    return diffs;
  };

  /**
   * Reduce the number of edits by eliminating operationally trivial equalities.
   * @param {!Array.<!DiffMatchPatch.Diff>} diffs Array of diff tuples.
   */
  DiffMatchPatch.prototype.diffCleanupEfficiency = function (diffs) {
    let changes, equalities, equalitiesLength, lastequality,
      pointer, preIns, preDel, postIns, postDel;
    changes = false;
    equalities = []; // Stack of indices where equalities are found.
    equalitiesLength = 0; // Keeping our own length var is faster in JS.
    /** @type {?string} */
    lastequality = null;

    // Always equal to diffs[equalities[equalitiesLength - 1]][1]
    pointer = 0; // Index of current position.

    // Is there an insertion operation before the last equality.
    preIns = false;

    // Is there a deletion operation before the last equality.
    preDel = false;

    // Is there an insertion operation after the last equality.
    postIns = false;

    // Is there a deletion operation after the last equality.
    postDel = false;
    while (pointer < diffs.length) {
      // Equality found.
      if (diffs[pointer][0] === DIFF_EQUAL) {
        if (diffs[pointer][1].length < 4 && (postIns || postDel)) {
          // Candidate found.
          equalities[equalitiesLength++] = pointer;
          preIns = postIns;
          preDel = postDel;
          lastequality = diffs[pointer][1];
        } else {
          // Not a candidate, and can never become one.
          equalitiesLength = 0;
          lastequality = null;
        }
        postIns = postDel = false;

        // An insertion or deletion.
      } else {
        if (diffs[pointer][0] === DIFF_DELETE) {
          postDel = true;
        } else {
          postIns = true;
        }

        /*
         * Five types to be split:
         * <ins>A</ins><del>B</del>XY<ins>C</ins><del>D</del>
         * <ins>A</ins>X<ins>C</ins><del>D</del>
         * <ins>A</ins><del>B</del>X<ins>C</ins>
         * <ins>A</del>X<ins>C</ins><del>D</del>
         * <ins>A</ins><del>B</del>X<del>C</del>
         */
        if (lastequality && ((preIns && preDel && postIns && postDel)
            || ((lastequality.length < 2)
            && (preIns + preDel + postIns + postDel) === 3))) {
          // Duplicate record.
          diffs.splice(
            equalities[equalitiesLength - 1],
            0,
            [DIFF_DELETE, lastequality]
          );

          // Change second copy to insert.
          diffs[equalities[equalitiesLength - 1] + 1][0] = DIFF_INSERT;
          equalitiesLength--; // Throw away the equality we just deleted;
          lastequality = null;
          if (preIns && preDel) {
            // No changes made which could affect previous entry, keep going.
            postIns = postDel = true;
            equalitiesLength = 0;
          } else {
            equalitiesLength--; // Throw away the previous equality.
            pointer = equalitiesLength > 0 ? equalities[equalitiesLength - 1] : -1;
            postIns = postDel = false;
          }
          changes = true;
        }
      }
      pointer++;
    }

    if (changes) {
      this.diffCleanupMerge(diffs);
    }
  };

  /**
   * Convert a diff array into a pretty HTML report.
   * @param {!Array.<!DiffMatchPatch.Diff>} diffs Array of diff tuples.
   * @param {integer} string to be beautified.
   * @return {string} HTML representation.
   */
  DiffMatchPatch.prototype.diffPrettyHtml = function (diffs) {
    const html = [];
    for (let x = 0; x < diffs.length; x++) {
      const op = diffs[x][0]; // Operation (insert, delete, equal)
      const data = diffs[x][1]; // Text of change.
      switch (op) {
        case DIFF_INSERT:
          html[x] = '<ins>' + escapeText(data) + '</ins>';
          break;
        case DIFF_DELETE:
          html[x] = '<del>' + escapeText(data) + '</del>';
          break;
        case DIFF_EQUAL:
          html[x] = '<span>' + escapeText(data) + '</span>';
          break;
      }
    }
    return html.join('');
  };

  /**
   * Determine the common prefix of two strings.
   * @param {string} text1 First string.
   * @param {string} text2 Second string.
   * @return {number} The number of characters common to the start of each
   *     string.
   */
  DiffMatchPatch.prototype.diffCommonPrefix = function (text1, text2) {
    let pointermid, pointermax, pointermin, pointerstart;

    // Quick check for common null cases.
    if (!text1 || !text2 || text1.charAt(0) !== text2.charAt(0)) {
      return 0;
    }

    // Binary search.
    // Performance analysis: https://neil.fraser.name/news/2007/10/09/
    pointermin = 0;
    pointermax = Math.min(text1.length, text2.length);
    pointermid = pointermax;
    pointerstart = 0;
    while (pointermin < pointermid) {
      if (text1.substring(pointerstart, pointermid) === text2.substring(pointerstart, pointermid)) {
        pointermin = pointermid;
        pointerstart = pointermin;
      } else {
        pointermax = pointermid;
      }
      pointermid = Math.floor((pointermax - pointermin) / 2 + pointermin);
    }
    return pointermid;
  };

  /**
   * Determine the common suffix of two strings.
   * @param {string} text1 First string.
   * @param {string} text2 Second string.
   * @return {number} The number of characters common to the end of each string.
   */
  DiffMatchPatch.prototype.diffCommonSuffix = function (text1, text2) {
    let pointermid, pointermax, pointermin, pointerend;

    // Quick check for common null cases.
    if (!text1
      || !text2
      || text1.charAt(text1.length - 1) !== text2.charAt(text2.length - 1)
    ) {
      return 0;
    }

    // Binary search.
    // Performance analysis: https://neil.fraser.name/news/2007/10/09/
    pointermin = 0;
    pointermax = Math.min(text1.length, text2.length);
    pointermid = pointermax;
    pointerend = 0;
    while (pointermin < pointermid) {
      if (
        text1.substring(text1.length - pointermid, text1.length - pointerend)
          === text2.substring(text2.length - pointermid, text2.length - pointerend)
      ) {
        pointermin = pointermid;
        pointerend = pointermin;
      } else {
        pointermax = pointermid;
      }
      pointermid = Math.floor((pointermax - pointermin) / 2 + pointermin);
    }
    return pointermid;
  };

  /**
   * Find the differences between two texts.  Assumes that the texts do not
   * have any common prefix or suffix.
   * @param {string} text1 Old string to be diffed.
   * @param {string} text2 New string to be diffed.
   * @param {boolean} checklines Speedup flag.  If false, then don't run a
   *     line-level diff first to identify the changed areas.
   *     If true, then run a faster, slightly less optimal diff.
   * @param {number} deadline Time when the diff should be complete by.
   * @return {!Array.<!DiffMatchPatch.Diff>} Array of diff tuples.
   * @private
   */
  DiffMatchPatch.prototype.diffCompute = function (text1, text2, checklines, deadline) {
    let diffs, longtext, shorttext, i, hm,
      text1A, text2A, text1B, text2B,
      midCommon, diffsA, diffsB;

    if (!text1) {
      // Just add some text (speedup).
      return [
        [DIFF_INSERT, text2]
      ];
    }

    if (!text2) {
      // Just delete some text (speedup).
      return [
        [DIFF_DELETE, text1]
      ];
    }

    longtext = text1.length > text2.length ? text1 : text2;
    shorttext = text1.length > text2.length ? text2 : text1;
    i = longtext.indexOf(shorttext);
    if (i !== -1) {
      // Shorter text is inside the longer text (speedup).
      diffs = [
        [DIFF_INSERT, longtext.substring(0, i)],
        [DIFF_EQUAL, shorttext],
        [DIFF_INSERT, longtext.substring(i + shorttext.length)]
      ];

      // Swap insertions for deletions if diff is reversed.
      if (text1.length > text2.length) {
        diffs[0][0] = diffs[2][0] = DIFF_DELETE;
      }
      return diffs;
    }

    if (shorttext.length === 1) {
      // Single character string.
      // After the previous speedup, the character can't be an equality.
      return [
        [DIFF_DELETE, text1],
        [DIFF_INSERT, text2]
      ];
    }

    // Check to see if the problem can be split in two.
    hm = this.diffHalfMatch(text1, text2);
    if (hm) {
      // A half-match was found, sort out the return data.
      text1A = hm[0];
      text1B = hm[1];
      text2A = hm[2];
      text2B = hm[3];
      midCommon = hm[4];

      // Send both pairs off for separate processing.
      diffsA = this.DiffMain(text1A, text2A, checklines, deadline);
      diffsB = this.DiffMain(text1B, text2B, checklines, deadline);

      // Merge the results.
      return diffsA.concat([
        [DIFF_EQUAL, midCommon]
      ], diffsB);
    }

    if (checklines && text1.length > 100 && text2.length > 100) {
      return this.diffLineMode(text1, text2, deadline);
    }

    return this.diffBisect(text1, text2, deadline);
  };

  /**
   * Do the two texts share a substring which is at least half the length of the
   * longer text?
   * This speedup can produce non-minimal diffs.
   * @param {string} text1 First string.
   * @param {string} text2 Second string.
   * @return {Array.<string>} Five element Array, containing the prefix of
   *     text1, the suffix of text1, the prefix of text2, the suffix of
   *     text2 and the common middle.  Or null if there was no match.
   * @private
   */
  DiffMatchPatch.prototype.diffHalfMatch = function (text1, text2) {
    let longtext, shorttext, dmp,
      text1A, text2B, text2A, text1B, midCommon,
      hm1, hm2, hm;

    longtext = text1.length > text2.length ? text1 : text2;
    shorttext = text1.length > text2.length ? text2 : text1;
    if (longtext.length < 4 || shorttext.length * 2 < longtext.length) {
      return null; // Pointless.
    }
    dmp = this; // 'this' becomes 'window' in a closure.

    /**
     * Does a substring of shorttext exist within longtext such that the substring
     * is at least half the length of longtext?
     * Closure, but does not reference any external variables.
     * @param {string} longtext Longer string.
     * @param {string} shorttext Shorter string.
     * @param {number} i Start index of quarter length substring within longtext.
     * @return {Array.<string>} Five element Array, containing the prefix of
     *     longtext, the suffix of longtext, the prefix of shorttext, the suffix
     *     of shorttext and the common middle.  Or null if there was no match.
     * @private
     */
    function diffHalfMatchI (longtext, shorttext, i) {
      let seed, j, bestCommon, prefixLength, suffixLength,
        bestLongtextA, bestLongtextB, bestShorttextA, bestShorttextB;

      // Start with a 1/4 length substring at position i as a seed.
      seed = longtext.substring(i, i + Math.floor(longtext.length / 4));
      j = -1;
      bestCommon = '';
      while ((j = shorttext.indexOf(seed, j + 1)) !== -1) {
        prefixLength = dmp.diffCommonPrefix(longtext.substring(i),
          shorttext.substring(j));
        suffixLength = dmp.diffCommonSuffix(longtext.substring(0, i),
          shorttext.substring(0, j));
        if (bestCommon.length < suffixLength + prefixLength) {
          bestCommon = shorttext.substring(j - suffixLength, j)
            + shorttext.substring(j, j + prefixLength);
          bestLongtextA = longtext.substring(0, i - suffixLength);
          bestLongtextB = longtext.substring(i + prefixLength);
          bestShorttextA = shorttext.substring(0, j - suffixLength);
          bestShorttextB = shorttext.substring(j + prefixLength);
        }
      }
      if (bestCommon.length * 2 >= longtext.length) {
        return [bestLongtextA, bestLongtextB,
          bestShorttextA, bestShorttextB, bestCommon
        ];
      } else {
        return null;
      }
    }

    // First check if the second quarter is the seed for a half-match.
    hm1 = diffHalfMatchI(longtext, shorttext,
      Math.ceil(longtext.length / 4));

    // Check again based on the third quarter.
    hm2 = diffHalfMatchI(longtext, shorttext,
      Math.ceil(longtext.length / 2));
    if (!hm1 && !hm2) {
      return null;
    } else if (!hm2) {
      hm = hm1;
    } else if (!hm1) {
      hm = hm2;
    } else {
      // Both matched.  Select the longest.
      hm = hm1[4].length > hm2[4].length ? hm1 : hm2;
    }

    // A half-match was found, sort out the return data.
    if (text1.length > text2.length) {
      text1A = hm[0];
      text1B = hm[1];
      text2A = hm[2];
      text2B = hm[3];
    } else {
      text2A = hm[0];
      text2B = hm[1];
      text1A = hm[2];
      text1B = hm[3];
    }
    midCommon = hm[4];
    return [text1A, text1B, text2A, text2B, midCommon];
  };

  /**
   * Do a quick line-level diff on both strings, then rediff the parts for
   * greater accuracy.
   * This speedup can produce non-minimal diffs.
   * @param {string} text1 Old string to be diffed.
   * @param {string} text2 New string to be diffed.
   * @param {number} deadline Time when the diff should be complete by.
   * @return {!Array.<!DiffMatchPatch.Diff>} Array of diff tuples.
   * @private
   */
  DiffMatchPatch.prototype.diffLineMode = function (text1, text2, deadline) {
    let a, diffs, linearray, pointer, countInsert,
      countDelete, textInsert, textDelete, j;

    // Scan the text on a line-by-line basis first.
    a = this.diffLinesToChars(text1, text2);
    text1 = a.chars1;
    text2 = a.chars2;
    linearray = a.lineArray;

    diffs = this.DiffMain(text1, text2, false, deadline);

    // Convert the diff back to original text.
    this.diffCharsToLines(diffs, linearray);

    // Eliminate freak matches (e.g. blank lines)
    this.diffCleanupSemantic(diffs);

    // Rediff any replacement blocks, this time character-by-character.
    // Add a dummy entry at the end.
    diffs.push([DIFF_EQUAL, '']);
    pointer = 0;
    countDelete = 0;
    countInsert = 0;
    textDelete = '';
    textInsert = '';
    while (pointer < diffs.length) {
      switch (diffs[pointer][0]) {
        case DIFF_INSERT:
          countInsert++;
          textInsert += diffs[pointer][1];
          break;
        case DIFF_DELETE:
          countDelete++;
          textDelete += diffs[pointer][1];
          break;
        case DIFF_EQUAL:

          // Upon reaching an equality, check for prior redundancies.
          if (countDelete >= 1 && countInsert >= 1) {
            // Delete the offending records and add the merged ones.
            diffs.splice(pointer - countDelete - countInsert,
              countDelete + countInsert);
            pointer = pointer - countDelete - countInsert;
            a = this.DiffMain(textDelete, textInsert, false, deadline);
            for (j = a.length - 1; j >= 0; j--) {
              diffs.splice(pointer, 0, a[j]);
            }
            pointer = pointer + a.length;
          }
          countInsert = 0;
          countDelete = 0;
          textDelete = '';
          textInsert = '';
          break;
      }
      pointer++;
    }
    diffs.pop(); // Remove the dummy entry at the end.

    return diffs;
  };

  /**
   * Find the 'middle snake' of a diff, split the problem in two
   * and return the recursively constructed diff.
   * See Myers 1986 paper: An O(ND) Difference Algorithm and Its Variations.
   * @param {string} text1 Old string to be diffed.
   * @param {string} text2 New string to be diffed.
   * @param {number} deadline Time at which to bail if not yet complete.
   * @return {!Array.<!DiffMatchPatch.Diff>} Array of diff tuples.
   * @private
   */
  DiffMatchPatch.prototype.diffBisect = function (text1, text2, deadline) {
    let text1Length, text2Length, maxD, vOffset, vLength,
      v1, v2, x, delta, front, k1start, k1end, k2start,
      k2end, k2Offset, k1Offset, x1, x2, y1, y2, d, k1, k2;

    // Cache the text lengths to prevent multiple calls.
    text1Length = text1.length;
    text2Length = text2.length;
    maxD = Math.ceil((text1Length + text2Length) / 2);
    vOffset = maxD;
    vLength = 2 * maxD;
    v1 = new Array(vLength);
    v2 = new Array(vLength);

    // Setting all elements to -1 is faster in Chrome & Firefox than mixing
    // integers and undefined.
    for (x = 0; x < vLength; x++) {
      v1[x] = -1;
      v2[x] = -1;
    }
    v1[vOffset + 1] = 0;
    v2[vOffset + 1] = 0;
    delta = text1Length - text2Length;

    // If the total number of characters is odd, then the front path will collide
    // with the reverse path.
    front = (delta % 2 !== 0);

    // Offsets for start and end of k loop.
    // Prevents mapping of space beyond the grid.
    k1start = 0;
    k1end = 0;
    k2start = 0;
    k2end = 0;
    for (d = 0; d < maxD; d++) {
      // Bail out if deadline is reached.
      if (Date.now() > deadline) {
        break;
      }

      // Walk the front path one step.
      for (k1 = -d + k1start; k1 <= d - k1end; k1 += 2) {
        k1Offset = vOffset + k1;
        if (k1 === -d || (k1 !== d && v1[k1Offset - 1] < v1[k1Offset + 1])) {
          x1 = v1[k1Offset + 1];
        } else {
          x1 = v1[k1Offset - 1] + 1;
        }
        y1 = x1 - k1;
        while (
          x1 < text1Length && y1 < text2Length
          && text1.charAt(x1) === text2.charAt(y1)
        ) {
          x1++;
          y1++;
        }
        v1[k1Offset] = x1;
        if (x1 > text1Length) {
          // Ran off the right of the graph.
          k1end += 2;
        } else if (y1 > text2Length) {
          // Ran off the bottom of the graph.
          k1start += 2;
        } else if (front) {
          k2Offset = vOffset + delta - k1;
          if (k2Offset >= 0 && k2Offset < vLength && v2[k2Offset] !== -1) {
            // Mirror x2 onto top-left coordinate system.
            x2 = text1Length - v2[k2Offset];
            if (x1 >= x2) {
              // Overlap detected.
              return this.diffBisectSplit(text1, text2, x1, y1, deadline);
            }
          }
        }
      }

      // Walk the reverse path one step.
      for (k2 = -d + k2start; k2 <= d - k2end; k2 += 2) {
        k2Offset = vOffset + k2;
        if (k2 === -d || (k2 !== d && v2[k2Offset - 1] < v2[k2Offset + 1])) {
          x2 = v2[k2Offset + 1];
        } else {
          x2 = v2[k2Offset - 1] + 1;
        }
        y2 = x2 - k2;
        while (
          x2 < text1Length
          && y2 < text2Length
          && text1.charAt(text1Length - x2 - 1) === text2.charAt(text2Length - y2 - 1)
        ) {
          x2++;
          y2++;
        }
        v2[k2Offset] = x2;
        if (x2 > text1Length) {
          // Ran off the left of the graph.
          k2end += 2;
        } else if (y2 > text2Length) {
          // Ran off the top of the graph.
          k2start += 2;
        } else if (!front) {
          k1Offset = vOffset + delta - k2;
          if (k1Offset >= 0 && k1Offset < vLength && v1[k1Offset] !== -1) {
            x1 = v1[k1Offset];
            y1 = vOffset + x1 - k1Offset;

            // Mirror x2 onto top-left coordinate system.
            x2 = text1Length - x2;
            if (x1 >= x2) {
              // Overlap detected.
              return this.diffBisectSplit(text1, text2, x1, y1, deadline);
            }
          }
        }
      }
    }

    // Diff took too long and hit the deadline or
    // number of diffs equals number of characters, no commonality at all.
    return [
      [DIFF_DELETE, text1],
      [DIFF_INSERT, text2]
    ];
  };

  /**
   * Given the location of the 'middle snake', split the diff in two parts
   * and recurse.
   * @param {string} text1 Old string to be diffed.
   * @param {string} text2 New string to be diffed.
   * @param {number} x Index of split point in text1.
   * @param {number} y Index of split point in text2.
   * @param {number} deadline Time at which to bail if not yet complete.
   * @return {!Array.<!DiffMatchPatch.Diff>} Array of diff tuples.
   * @private
   */
  DiffMatchPatch.prototype.diffBisectSplit = function (text1, text2, x, y, deadline) {
    let text1a, text1b, text2a, text2b, diffs, diffsb;
    text1a = text1.substring(0, x);
    text2a = text2.substring(0, y);
    text1b = text1.substring(x);
    text2b = text2.substring(y);

    // Compute both diffs serially.
    diffs = this.DiffMain(text1a, text2a, false, deadline);
    diffsb = this.DiffMain(text1b, text2b, false, deadline);

    return diffs.concat(diffsb);
  };

  /**
   * Reduce the number of edits by eliminating semantically trivial equalities.
   * @param {!Array.<!DiffMatchPatch.Diff>} diffs Array of diff tuples.
   */
  DiffMatchPatch.prototype.diffCleanupSemantic = function (diffs) {
    let changes = false;
    let equalities = []; // Stack of indices where equalities are found.
    let equalitiesLength = 0; // Keeping our own length var is faster in JS.
    /** @type {?string} */
    let lastequality = null;

    // Always equal to diffs[equalities[equalitiesLength - 1]][1]
    let pointer = 0; // Index of current position.

    // Number of characters that changed prior to the equality.
    let lengthInsertions1 = 0;
    let lengthDeletions1 = 0;

    // Number of characters that changed after the equality.
    let lengthInsertions2 = 0;
    let lengthDeletions2 = 0;
    while (pointer < diffs.length) {
      if (diffs[pointer][0] === DIFF_EQUAL) { // Equality found.
        equalities[equalitiesLength++] = pointer;
        lengthInsertions1 = lengthInsertions2;
        lengthDeletions1 = lengthDeletions2;
        lengthInsertions2 = 0;
        lengthDeletions2 = 0;
        lastequality = diffs[pointer][1];
      } else { // An insertion or deletion.
        if (diffs[pointer][0] === DIFF_INSERT) {
          lengthInsertions2 += diffs[pointer][1].length;
        } else {
          lengthDeletions2 += diffs[pointer][1].length;
        }

        // Eliminate an equality that is smaller or equal to the edits on both
        // sides of it.
        if (lastequality
          && (lastequality.length <= Math.max(lengthInsertions1, lengthDeletions1))
          && (lastequality.length <= Math.max(lengthInsertions2, lengthDeletions2))
        ) {
          // Duplicate record.
          diffs.splice(
            equalities[equalitiesLength - 1],
            0,
            [DIFF_DELETE, lastequality]
          );

          // Change second copy to insert.
          diffs[equalities[equalitiesLength - 1] + 1][0] = DIFF_INSERT;

          // Throw away the equality we just deleted.
          equalitiesLength--;

          // Throw away the previous equality (it needs to be reevaluated).
          equalitiesLength--;
          pointer = equalitiesLength > 0 ? equalities[equalitiesLength - 1] : -1;

          // Reset the counters.
          lengthInsertions1 = 0;
          lengthDeletions1 = 0;
          lengthInsertions2 = 0;
          lengthDeletions2 = 0;
          lastequality = null;
          changes = true;
        }
      }
      pointer++;
    }

    // Normalize the diff.
    if (changes) {
      this.diffCleanupMerge(diffs);
    }

    let deletion, insertion, overlapLength1, overlapLength2;

    // Find any overlaps between deletions and insertions.
    // e.g: <del>abcxxx</del><ins>xxxdef</ins>
    //   -> <del>abc</del>xxx<ins>def</ins>
    // e.g: <del>xxxabc</del><ins>defxxx</ins>
    //   -> <ins>def</ins>xxx<del>abc</del>
    // Only extract an overlap if it is as big as the edit ahead or behind it.
    pointer = 1;
    while (pointer < diffs.length) {
      if (
        diffs[pointer - 1][0] === DIFF_DELETE
        && diffs[pointer][0] === DIFF_INSERT
      ) {
        deletion = diffs[pointer - 1][1];
        insertion = diffs[pointer][1];
        overlapLength1 = this.diffCommonOverlap(deletion, insertion);
        overlapLength2 = this.diffCommonOverlap(insertion, deletion);
        if (overlapLength1 >= overlapLength2) {
          if (overlapLength1 >= deletion.length / 2 || overlapLength1 >= insertion.length / 2) {
            // Overlap found.  Insert an equality and trim the surrounding edits.
            diffs.splice(
              pointer,
              0,
              [DIFF_EQUAL, insertion.substring(0, overlapLength1)]
            );
            diffs[pointer - 1][1] =
              deletion.substring(0, deletion.length - overlapLength1);
            diffs[pointer + 1][1] = insertion.substring(overlapLength1);
            pointer++;
          }
        } else {
          if (overlapLength2 >= deletion.length / 2 || overlapLength2 >= insertion.length / 2) {
            // Reverse overlap found.
            // Insert an equality and swap and trim the surrounding edits.
            diffs.splice(
              pointer,
              0,
              [DIFF_EQUAL, deletion.substring(0, overlapLength2)]
            );

            diffs[pointer - 1][0] = DIFF_INSERT;
            diffs[pointer - 1][1] = insertion.substring(0, insertion.length - overlapLength2);
            diffs[pointer + 1][0] = DIFF_DELETE;
            diffs[pointer + 1][1] = deletion.substring(overlapLength2);
            pointer++;
          }
        }
        pointer++;
      }
      pointer++;
    }
  };

  /**
   * Determine if the suffix of one string is the prefix of another.
   * @param {string} text1 First string.
   * @param {string} text2 Second string.
   * @return {number} The number of characters common to the end of the first
   *     string and the start of the second string.
   * @private
   */
  DiffMatchPatch.prototype.diffCommonOverlap = function (text1, text2) {
    // Cache the text lengths to prevent multiple calls.
    const text1Length = text1.length;
    const text2Length = text2.length;

    // Eliminate the null case.
    if (text1Length === 0 || text2Length === 0) {
      return 0;
    }

    // Truncate the longer string.
    if (text1Length > text2Length) {
      text1 = text1.substring(text1Length - text2Length);
    } else if (text1Length < text2Length) {
      text2 = text2.substring(0, text1Length);
    }
    const textLength = Math.min(text1Length, text2Length);

    // Quick check for the worst case.
    if (text1 === text2) {
      return textLength;
    }

    // Start by looking for a single character match
    // and increase length until no match is found.
    // Performance analysis: https://neil.fraser.name/news/2010/11/04/
    let best = 0;
    let length = 1;
    while (true) {
      const pattern = text1.substring(textLength - length);
      const found = text2.indexOf(pattern);
      if (found === -1) {
        return best;
      }
      length += found;
      if (
        found === 0
        || text1.substring(textLength - length) === text2.substring(0, length)
      ) {
        best = length;
        length++;
      }
    }
  };

  /**
   * Split two texts into an array of strings.  Reduce the texts to a string of
   * hashes where each Unicode character represents one line.
   * @param {string} text1 First string.
   * @param {string} text2 Second string.
   * @return {{chars1: string, chars2: string, lineArray: !Array.<string>}}
   *     An object containing the encoded text1, the encoded text2 and
   *     the array of unique strings.
   *     The zeroth element of the array of unique strings is intentionally blank.
   * @private
   */
  DiffMatchPatch.prototype.diffLinesToChars = function (text1, text2) {
    let lineArray = []; // E.g. lineArray[4] === 'Hello\n'
    let lineHash = {}; // E.g. lineHash['Hello\n'] === 4

    // '\x00' is a valid character, but various debuggers don't like it.
    // So we'll insert a junk entry to avoid generating a null character.
    lineArray[0] = '';

    /**
     * Split a text into an array of strings.  Reduce the texts to a string of
     * hashes where each Unicode character represents one line.
     * Modifies linearray and linehash through being a closure.
     * @param {string} text String to encode.
     * @return {string} Encoded string.
     * @private
     */
    function diffLinesToCharsMunge (text) {
      let chars = '';

      // Walk the text, pulling out a substring for each line.
      // text.split('\n') would would temporarily double our memory footprint.
      // Modifying text would create many large strings to garbage collect.
      let lineStart = 0;
      let lineEnd = -1;

      // Keeping our own length variable is faster than looking it up.
      let lineArrayLength = lineArray.length;
      while (lineEnd < text.length - 1) {
        lineEnd = text.indexOf('\n', lineStart);
        if (lineEnd === -1) {
          lineEnd = text.length - 1;
        }
        let line = text.substring(lineStart, lineEnd + 1);
        lineStart = lineEnd + 1;

        if (hasOwn.call(lineHash, line)) {
          chars += String.fromCharCode(lineHash[line]);
        } else {
          chars += String.fromCharCode(lineArrayLength);
          lineHash[line] = lineArrayLength;
          lineArray[lineArrayLength++] = line;
        }
      }
      return chars;
    }

    const chars1 = diffLinesToCharsMunge(text1);
    const chars2 = diffLinesToCharsMunge(text2);
    return {
      chars1: chars1,
      chars2: chars2,
      lineArray: lineArray
    };
  };

  /**
   * Rehydrate the text in a diff from a string of line hashes to real lines of
   * text.
   * @param {!Array.<!DiffMatchPatch.Diff>} diffs Array of diff tuples.
   * @param {!Array.<string>} lineArray Array of unique strings.
   * @private
   */
  DiffMatchPatch.prototype.diffCharsToLines = function (diffs, lineArray) {
    for (let x = 0; x < diffs.length; x++) {
      const chars = diffs[x][1];
      const text = [];
      for (let y = 0; y < chars.length; y++) {
        text[y] = lineArray[chars.charCodeAt(y)];
      }
      diffs[x][1] = text.join('');
    }
  };

  /**
   * Reorder and merge like edit sections.  Merge equalities.
   * Any edit section can move as long as it doesn't cross an equality.
   * @param {!Array.<!DiffMatchPatch.Diff>} diffs Array of diff tuples.
   */
  DiffMatchPatch.prototype.diffCleanupMerge = function (diffs) {
    diffs.push([DIFF_EQUAL, '']); // Add a dummy entry at the end.
    let pointer = 0;
    let countDelete = 0;
    let countInsert = 0;
    let textDelete = '';
    let textInsert = '';

    while (pointer < diffs.length) {
      switch (diffs[pointer][0]) {
        case DIFF_INSERT:
          countInsert++;
          textInsert += diffs[pointer][1];
          pointer++;
          break;
        case DIFF_DELETE:
          countDelete++;
          textDelete += diffs[pointer][1];
          pointer++;
          break;
        case DIFF_EQUAL:

          // Upon reaching an equality, check for prior redundancies.
          if (countDelete + countInsert > 1) {
            if (countDelete !== 0 && countInsert !== 0) {
              // Factor out any common prefixes.
              let commonlength = this.diffCommonPrefix(textInsert, textDelete);
              if (commonlength !== 0) {
                if (
                  (pointer - countDelete - countInsert) > 0
                  && diffs[pointer - countDelete - countInsert - 1][0] === DIFF_EQUAL
                ) {
                  diffs[pointer - countDelete - countInsert - 1][1] +=
                    textInsert.substring(0, commonlength);
                } else {
                  diffs.splice(0, 0, [DIFF_EQUAL,
                    textInsert.substring(0, commonlength)
                  ]);
                  pointer++;
                }
                textInsert = textInsert.substring(commonlength);
                textDelete = textDelete.substring(commonlength);
              }

              // Factor out any common suffixies.
              commonlength = this.diffCommonSuffix(textInsert, textDelete);
              if (commonlength !== 0) {
                diffs[pointer][1] = textInsert.substring(textInsert.length - commonlength) + diffs[pointer][1];
                textInsert = textInsert.substring(0, textInsert.length - commonlength);
                textDelete = textDelete.substring(0, textDelete.length - commonlength);
              }
            }

            // Delete the offending records and add the merged ones.
            if (countDelete === 0) {
              diffs.splice(
                pointer - countInsert,
                countDelete + countInsert,
                [DIFF_INSERT, textInsert]
              );
            } else if (countInsert === 0) {
              diffs.splice(
                pointer - countDelete,
                countDelete + countInsert,
                [DIFF_DELETE, textDelete]
              );
            } else {
              diffs.splice(
                pointer - countDelete - countInsert,
                countDelete + countInsert,
                [DIFF_DELETE, textDelete], [DIFF_INSERT, textInsert]
              );
            }
            pointer = pointer - countDelete - countInsert
              + (countDelete ? 1 : 0)
              + (countInsert ? 1 : 0)
              + 1;
          } else if (pointer !== 0 && diffs[pointer - 1][0] === DIFF_EQUAL) {
            // Merge this equality with the previous one.
            diffs[pointer - 1][1] += diffs[pointer][1];
            diffs.splice(pointer, 1);
          } else {
            pointer++;
          }
          countInsert = 0;
          countDelete = 0;
          textDelete = '';
          textInsert = '';
          break;
      }
    }
    if (diffs[diffs.length - 1][1] === '') {
      diffs.pop(); // Remove the dummy entry at the end.
    }

    // Second pass: look for single edits surrounded on both sides by equalities
    // which can be shifted sideways to eliminate an equality.
    // e.g: A<ins>BA</ins>C -> <ins>AB</ins>AC
    let changes = false;
    pointer = 1;

    // Intentionally ignore the first and last element (don't need checking).
    while (pointer < diffs.length - 1) {
      if (
        diffs[pointer - 1][0] === DIFF_EQUAL
        && diffs[pointer + 1][0] === DIFF_EQUAL
      ) {
        const diffPointer = diffs[pointer][1];
        const position = diffPointer.substring(
          diffPointer.length - diffs[pointer - 1][1].length
        );

        // This is a single edit surrounded by equalities.
        if (position === diffs[pointer - 1][1]) {
          // Shift the edit over the previous equality.
          diffs[pointer][1] = diffs[pointer - 1][1] + diffs[pointer][1].substring(0, diffs[pointer][1].length - diffs[pointer - 1][1].length);
          diffs[pointer + 1][1] = diffs[pointer - 1][1] + diffs[pointer + 1][1];
          diffs.splice(pointer - 1, 1);
          changes = true;
        } else if (diffPointer.substring(0, diffs[pointer + 1][1].length) === diffs[pointer + 1][1]) {
          // Shift the edit over the next equality.
          diffs[pointer - 1][1] += diffs[pointer + 1][1];
          diffs[pointer][1] =
            diffs[pointer][1].substring(diffs[pointer + 1][1].length) + diffs[pointer + 1][1];
          diffs.splice(pointer + 1, 1);
          changes = true;
        }
      }
      pointer++;
    }

    // If shifts were made, the diff needs reordering and another shift sweep.
    if (changes) {
      this.diffCleanupMerge(diffs);
    }
  };

  export default function diff (o, n) {
    let diff, output, text;
    diff = new DiffMatchPatch();
    output = diff.DiffMain(o, n);
    diff.diffCleanupEfficiency(output);
    text = diff.diffPrettyHtml(output);

    return text;
  }
